Abstract
The assembly of magnetic cores into regular structures may notably influence the properties displayed by a magnetic colloid. Here, key synthesis parameters driving the self‐assembly process capable of organizing colloidal magnetic cores into highly regular and reproducible multi‐core nanoparticles are determined. In addition, a self‐consistent picture that explains the collective magnetic properties exhibited by these complex assemblies is achieved through structural, colloidal, and magnetic means. For this purpose, different strategies to obtain flower‐shaped iron oxide assemblies in the size range 25–100 nm are examined. The routes are based on the partial oxidation of Fe(OH)2, polyol‐mediated synthesis or the reduction of iron acetylacetonate. The nanoparticles are functionalized either with dextran, citric acid, or alternatively embedded in polystyrene and their long‐term stability is assessed. The core size is measured, calculated, and modeled using both structural and magnetic means, while the Debye model and multi‐core extended model are used to study interparticle interactions. This is the first step toward standardized protocols of synthesis and characterization of flower‐shaped nanoparticles.
Highlights
Despite the progress in colloidal self-assembly of organic[1] or inorganic[2,3] building blocks to form close-packed structures such as colloidal crystals,[4] there are only a few reports of controlled assembly of ordered nanoparticles in suspension.[5]
Nanoflowers produced in this way have a mean diameter of 46 nm and they are composed of 7 nm cores that are loosely packed together (Figure 1)
We observe that flower-shaped nanoparticles produced in this way have a poorly defined size, shape and broad size distribution, probably due to the poor capping effect of dextran hydroxyl groups, which are attached through hydrogen bonds to the iron oxide particle surface.[39]
Summary
Despite the progress in colloidal self-assembly of organic[1] or inorganic[2,3] building blocks to form close-packed structures such as colloidal crystals,[4] there are only a few reports of controlled assembly of ordered nanoparticles in suspension.[5]. Exchange interactions between cores of a multi-core particle may lead to the so-called “superferrimagnetic” behaviour,[13] exhibiting large magnetic moment and weak remanence in zero field, and having low tendency to form agglomerates. We analyse the key synthesis parameters driving the self-assembly process capable of organizing colloidal magnetic cores into highly regular and reproducible multi-core nanoparticles showing the so called “superferrimagnetic state” due to exchange interactions. A key parameter to understand the behaviour of the colloid is the degree of fusion of the cores within the nanoflowers, whether they are in direct contact and if so, if they share crystalline alignment.[31] Secondly, we analyse the interparticle interactions,[32] which are minimized by steric and/or electrostatic repulsion due to the surface coatings (dextran, citric acid) or alternatively by embedding the cores on surfactant stabilized polystyrene beads. N-methyl diethanolamine, HPH process stands for high-pressure homogenization coating and ESE process stands for emulsion solvent evaporation
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